1. Field of the Invention
The invention relates to field of disk drives and more particularly to the writing of servo tracks onto the disks during manufacture.
2. Description of the Related Art
Disk drive servo tracks are today written onto a blank magnetic disk after the disk drive has been substantially assembled. While there are many known methods for writing servo tracks onto a blank magnetic disk, one of the most common methods include the use of a laser interferometer to control a picker that attaches to an actuator arm. The picker steps the arms across the disk. A head, mounted on the arm writes the servo patterns.
As disk drive areal density continues to increase, pre-patterned servo fields are proposed. These fields may be formed by high precision lithographic methods. See, e.g., U.S. Pat. No. 6,331,364 B1. A mask exposes areas to be demagnetized. An ion bombardment through the mask reduces the coercivity of the disk magnetic material, making it unable to retain magnetization. Arranging magnetized and demagnetized areas forms a servo pattern.
A problem occurs when pre-patterned media is mounted onto a spindle. It is typically not possible to precisely center the media patterns with respect to the axis of rotation of the spindle. This results in the center of the servo patterns being spaced a small distance from the axis of rotation of the spindle. As a result, the servo patterns form an eccentric “circle” about the spindle.
FIG. 1 illustrates the problem. It shows, greatly exaggerated, a preformatted disk 10 mounted on a spindle 12 with the center of the preformatted servo pattern 14 (comprising a plurality of spaced servo bursts 20) offset from the center of the spindle 12. Also shown in the figure is a hypothetical actuator arm 16, upon the distal end 18 of which is typically mounted a magnetic transducer head (not shown in figure) that reads the magnetic information from the disk 10 as it rotates.
Servo patterns can be of many varieties all of which enable the servo system of a disk drive to “follow” a track of information, while either reading or writing to it. The most common servo pattern in use in today's disc drives is the so-call “quadrature” servo burst. See e.g., FIG. 3 of U.S. Pat. No. 5,760,990, for a typical quadrature pattern. FIG. 2 illustrates such a “quadrature” servo burst 20. In FIG. 2, the servo burst is comprised of three separate areas, labeled in the figure as Gain, PS1 and PS2. These areas follow each other in the “track” direction. Each is comprised of a plurality of radially aligned patterns. One of patterns, labeled Gain, operates to calibrate the gain of a servo read circuit. The Gain patterns are radially continuous. The other two patterns are the servo bursts themselves, PS1 and PS2. The servo bursts, PS1 and PS2, patterns are not continuous. They are rather formed of two sets of adjacent patterns. A magnetic head reading, for example, pattern PS1, will generate a “null” signal when it is aligned equally over each of the offset patterns of PS1. The same is true when the head tracks down the center between the two offset patterns of PS2.
The PS1 and PS2 patterns in turn are radially offset from each other by 90 degrees. This is best illustrated by reference to FIG. 3, a chart showing the PES signals read from each of these patterns as the head is traversed in the radial direction. Each of the points in FIG. 3 represents the signal generated by an entire PS1 or PS2 burst at a particular radial location as that pattern passes under a magnetic head. The PS 1 and PS 2 waveforms, although they look very much like a sine waves, contain second and higher order components.
A PES signal, generated by either PS1 or PS2 in the track dimension, is illustrated in FIG. 4. Assuming disk to spindle eccentricity was not too severe, i.e., eccentricity did not exceed one track, the track-wise PES signal is similarly a sine like periodic waveform. The signal varies from a sine wave because it contains components attributable to the repeatable run out of the servo burst pattern vs. the axis rotation of the spindle. Other than for eccentricity, these errors are caused by errors in the formation of the servo patterns. The errors are typically caused by mechanical vibrations, air turbulence, or electrical noise if the patterns are written magnetically. Poor lithographic processes cause the errors if lithographic processes write the servo patterns.
When the degree of misalignment between the center of the servo patterns 14 and the center of rotation of a spindle 12 exceeds one track, the PES signal may exhibit more than one period as the disk spins one revolution and the transducer is held fixed, i.e., “stationary.” The number of repetitions is according to the number of servo tracks that pass under the stationery transducer as the disk rotates. For example, if the eccentricity of the disk is only slightly greater than one track, the PES read back signal could appear as illustrated in FIG. 5. The peaks correspond to track “center” as its passes under the transducer during one revolution.
Today's disk drive track density approaches 100,000 tracks per inch. In this environment, the number of tracks offset between a servo pattern center and the center of rotation of the spindle will typically exceed 10 or more tracks. The PES signal read from a stationery transducer will contain a large number of peaks. An exemplary pattern is illustrated in FIG. 6.
In the figure, the y-coordinate represents PS1 read back amplitude (in arbitrary units). The x-coordinate represents the sector number of a servo burst. The illustrated disk drive track has 250 servo sectors.
The 250 points are fairly scattered and show no discernible pattern.
When a disc drive has pre-patterned servo, it is important to characterize and control the degree of servo pattern irregularity (the repeatable and non-repeatable errors) so that a disk drive may function. Excessive irregularity can prevent the disk drive from tracking at all.
In prior systems, repeatable runout compensation schemes were developed to aid in tracking disks having a high degree irregularity. Repeatable runout was measured and feed forward into the servo-positioning signal during operation. One such system for measuring repeatable run out is described in U.S. Pat. No. 6,310,742 B1. Such prior systems for measuring, however, will not work or take far too much time to measure when the servo track eccentricity (runout) far exceeds one track such as one finds with pre-patterned servo media.
Therefore, there is a need to develop a system to measure servo track repeatable runout of pre-formatted servo media, both for the purpose of disc media quality control and to provide for the purposes of “feeding forward” the runout into the disk drive's servo system.